Optical sensors are commonly used for a variety of functions including detecting skewed or multiple picked media items within a media handler in a self-service terminal. One type of self-service terminal that relies on an accurate optical sensor is an Automated Teller Machine (ATM). An ATM typically includes a cash dispenser having a pick unit for picking individual banknotes and conveying these banknotes to a banknote transport mechanism for delivery to an ATM customer.
ATMs have used a variety of different optical and non-optical detectors to detect banknote skew and multiple picked banknotes. These sensors are sometimes referred to as note thickness sensors (NTS). One particular type of NTS relies on compensated opacity, and is described in U.S. Pat. No. 7,049,572.
The design of the NTS in U.S. Pat. No. 7,049,572 uses a variable intensity white light LED source and a co-operating phototransistor receiver. The source and receiver are located on opposing sides of a banknote transport path. The LED is driven by an increasing current until a pre-determined threshold is reached by the phototransistor, for example, 3.0V. When a banknote blocks the light path, the LED drive current is increased until the phototransistor output again reaches the pre-determined threshold. The greater the number of banknotes present, the greater the LED current required before the phototransistor receiver reaches the threshold. The current required to reach the threshold is used to ascertain the number of banknotes present.
This type of NTS relies on an analogue circuit to ensure that the sensor output is constant. One disadvantage of this arrangement is that a powerful light source (LED) is required that has a high energy consumption and requires expensive heat sinks. Furthermore, the spectral response of the LED changes as its temperature rises, leading to a reduction in output intensity for a fixed current. This means that the value of the drive current is not an accurate measure for ascertaining the number of banknotes present. This necessitates adding a further phototransistor receiver to detect the actual output intensity from the LED, and using this actual value to ascertain the number of banknotes present. This type of NTS is accordingly relatively expensive.